1. Field of the Invention
The present invention relates to a power supply system and to a method for charging at least one energy storage cell serving as an energy store for a DC link in a power supply system.
2. Description of the Related Art
It is becoming apparent that, in future, electronic systems that combine new energy storage technologies with electric drive technology will be increasingly used both in stationary applications, such as, for example, wind power installations, and in vehicles, such as hybrid or electric vehicles. In conventional applications, an electric machine which, for example, is in the form of a rotating-field machine is controlled by way of a converter in the form of an inverter. Characteristic of such systems is a so-called DC link via which an energy store, usually a battery, is connected to the DC voltage side of the inverter. In order to be able to meet the requirements in terms of output and power for a particular application, a plurality of battery cells are connected in series. Since the current provided by such an energy store has to flow through all the battery cells limited current, battery cells are often additionally connected in parallel in order to increase the maximum current.
As well as providing a high total voltage, the series connection of a plurality of battery cells brings with it the problem that the entire energy store fails if a single battery cell fails, since battery current is then no longer able to flow. Such a failure of the energy store may lead to failure of the entire system. In the case of a vehicle, failure of the drive battery may lead to the vehicle being “stranded”. In other applications, such as, for example, rotor blade adjustment in wind power installations, this may even give rise to hazardous situations under adverse general conditions, for example in a strong wind. For that reason, a constant objective is a high reliability of the energy store, “reliability” meaning the ability of a system to operate for a given time without any faults.
The German Patent Applications DE 10 2010 027857 and DE 10 2010 027861 describe batteries having a plurality of battery module strings that are directly connectable to an electric machine. The battery module strings have a plurality of battery modules connected in series, each battery module having at least one battery cell and an associated controllable coupling unit which allows the respective battery module string to be interrupted or allows the respectively associated at least one battery cell to be bridged or allows the respectively associated at least one battery cell to be connected into the respective battery module string, in dependence on control signals. By suitable actuation of the coupling units, for example with the aid of pulse width modulation, suitable phase signals may also be provided for controlling the electric machine, so that a separate pulse inverter may be dispensed with. The pulse inverter required for controlling the electric machine is therefore integrated, as it were, in the battery. For the purposes of disclosure, those two earlier Applications are also incorporated in their entirety in the present Application.
In contrast to conventional systems, in which an electric machine is controlled by way of an inverter and is supplied with electrical energy by an electrical energy store that is separate therefrom, a constant DC voltage is not available in the case of the batteries described in the German Applications DE 10 2010 027857 and DE 10 2010 027861, and therefore such batteries cannot be readily integrated into conventional power supply systems, such as, for example, on-board systems of an electric vehicle or a hybrid vehicle.